The present invention relates to a process for catalytic autothermal steam reforming of alcohols having two or more carbon atoms, in particular ethanol, by directing a preheated educt or reactant mixture of the alcohols, oxygen and water or steam over a catalyst.
For the production of hydrogen, as is known, that alcohols can be converted to hydrogen, carbon monoxide and carbon dioxide in the presence of steam and a suitable catalyst. The reaction is highly endothermic and proceeds for example according to the following reaction equation: The so-called steam to carbon ratio S/C is characteristic of this reaction. In equation (1), S/C equals 0.5.
Another known method for producing hydrogen is catalytic partial oxidation (CPO). During CPO, especially hydrocarbons are converted to carbon monoxide and hydrogen in the presence of oxygen and a catalyst, for example according to reaction equation (2). In the case of alcohols, the energetic situation is different; for instance, in the case of ethanol, pure partial oxidation is endothermic, i.e. a process that will not proceed without energy input (3a). If the amount of oxygen is increased (3b-3c), a selective catalyst is necessary which preferably oxidizes CO and not hydrogen. An important parameter of partial oxidation is the air coefficient λ, which is defined as the ratio between the number of moles of oxygen used to the number of moles of oxygen required for complete oxidation (see reaction equation (4)): 
The present invention relates to another method of hydrogen production, the so-called catalytic autothermal steam reforming. This process combines catalytic partial oxidation and steam reforming, wherein the exothermic oxidation supplies the necessary reaction heat for the subsequent endothermic steam reforming. For this purpose, the educt or reactant mixture can be preheated to a preheat temperature. At the temperature prevailing at the reactor outlet, the product mixture is in the thermodynamic equilibrium of the water-gas shift reaction. Autothermal steam reforming combines the advantages of catalytic (partial) oxidation (good start-up behavior) and steam reforming (high hydrogen yields).
A catalyst for use in an autothermal reformer has been disclosed in the art. This catalyst comprises 0.01% to 6% rhodium and 10 to 35% calcium oxide on an alumina support, which is furthermore promoted with 3 to 15% magnesium. The catalyst is used in the form of pellets and in particular has a low tendency to coke at low oxygen/carbon ratios. An example of a typical catalyst system for autothermal reforming comprises an iron oxide catalyst for partial oxidation for about one-third of its length, and the described rhodium catalyst for about two-thirds of its length.
A bifunctional catalyst for the partial oxidation of hydrocarbons has also been disclosed in the art. This catalyst exhibits dehydrogenation activity for dehydrogenating the hydrocarbons and is capable of selectively oxidizing the hydrocarbon chain. The dehydrogenation activity is provided by metals of the eighth group of the periodic table, while selective oxidation is effected by ionized oxygen. The sources for the ionized oxygen are oxides that crystallize in a fluorite or perovskite structure, such as for example zirconium oxide, cerium oxide, bismuth oxide etc. An example of a preferred catalyst is Pt/CeGdO. This is used in the form of pellets with diameters of 1.125 to 1.5 inches.
It in known in that art that a process for the catalytic production of hydrogen by means of a self-sustaining partial oxidation and steam reforming of hydrocarbons, wherein a mixture of the hydrocarbons and an oxygen-containing gas and optionally steam is reacted in the presence of a catalyst comprising rhodium dispersed on a support, which contains cerium and zirconium as cations. The catalyst is used in granular form.
It is also known in the art that a process and an apparatus for autothermal reforming of hydrocarbons wherein the fuel is added to a two-stage reformer via a feeding unit. In a heat exchanger in countercurrent and in a heat-exchanging manner, the resulting reformate is fed to the educt or reactant of the reforming process, which is led from the outside to the inside. The fuel added via the feeding unit, together with the educt or reactant, is directly provided to the reaction zone comprising a catalyst, where the combustion and reforming or catalysis is carried out. The reformer comprises a honeycomb carrier coated with a catalyst in an upper part, and a bed coated with catalyst in a lower part. A honeycomb carrier can also be used instead of the bed.
Catalytic autothermal steam reforming of alcohols seems to be a suitable process for the production of hydrogen in a vehicle powered by fuel cells since by means of this process the hydrogen needed to operate the fuel cells can be obtained from e.g. renewable resources such as bio-ethanol, which in some countries is already used in conventional combustion engines. For this field of application, the hydrogen productivity is of decisive importance. It can either be expressed based on the volume of catalyst, see equation (5), or on the mass of the employed noble metal, see equation (6):                               P          Kat                =                                            V              H2                                                      V                Kat                            ·              t                                ⁡                      [                                          Nm                3                                                              1                  Kat                                ·                h                                      ]                                              (        5        )                                          P          EM                =                                            V              H2                                                      M                EM                            ·              t                                ⁡                      [                                          Nm                3                                                              g                  EM                                ·                h                                      ]                                              (        6        )                PKat: Hydrogen productivity based on the volume VKat of the catalyst    PEM: Hydrogen productivity based on the mass of the noble metal    VH2: Volume of hydrogen produced under standard conditions    t: Time
One problem in connection with the use of alcohols for the production of hydrogen by means of autothermal reforming is the fact that for providing the alcohols, the existing infrastructure in place for gasoline and diesel fuel is used, i.e. the biological alcohol is transported in the same truck-trailers and stored in the same tanks as the other engine fuels so that a contamination of the alcohols by gasoline and diesel fuel cannot be prevented. Examinations of the purity of alcohols stored in vessels in which gasoline or diesel fuel had been previously stored have shown that these alcohols may contain between 0.5 and 10 wt.-% of these hydrocarbons, based on the total weight of alcohols and hydrocarbons. Usually, the contamination of the alcohols by hydrocarbons lies between 0.5 and 5 wt.-%.
Based on the forgoing, there is a need in the art for a process for catalytic autothermal steam reforming that allows the simultaneous reforming of alcohols and hydrocarbons with a very high hydrogen productivity, which is well suited for use in mobile systems.